Castle Karstaag Caverns is an ice cave on the north-northwestern coast of Solstheim, inhabited by rieklings.

It is found southeast of Castle Karstaag Ruins, west-northwest of Saering's Watch. The path down to the entrance is from the west-southwest. It is above a tall waterfall that plunges to the sea, at an inlet on the north coast of Solstheim, just after two streams join, one of which flows from the entrance to the caverns.

Upon entering you are in a tunnel with a stream flowing down the right-hand side and a path up to the left. If you turn around as soon as you enter there is a snowberry bush on the far side of the stream. The path forwards heads west and passes an ice pillar a few paces ahead. At the end of the tunnel is a short waterfall dropping from above and an opening to the south giving you a view of a huge cavern with three bridges crossing in front of you at different levels. Several rieklings can be seen running across the bridges. Under the bridges is a lake of water and on the left is Esmond Tyne lying dead on a patch of ice with a lit lantern beside him. If you are here for a quest for Glover Mallory then this is the target of that quest.

If you dive into the water and head due south you will eventually find an apprentice-locked chest with a large coin purse and a leveled greatsword beside it. A method of waterbreathing and a light spell is recommended. There is also an ice ramp leading down to the water from the right-hand side of the lowest bridge, opposite the body of Esmond Tyne. From the bottom of the ramp you can see up the east side of the cavern and see riekling camps at various levels.

At the top of the ramp is the bridge heading east across to the first camp and west into a narrow tunnel with a ledge to the right of it with two rieklings. One riekling patrols back and forth across the bridge, while a second wanders among the tents—periodically stopping above Esmond Tyne—and a third stands on top of a lookout platform at the back of the camp to the east.

Across the bridge there is a tent on the right-hand side, with a random dagger inside. Following the path to the rear you reach the lookout platform, which has two coin purses at the top, one large and one medium. To the left of this is a riekling hut and beside this is another tent with two barrels in front, one of which contains ingredients with a knapsack beside it.

Recross the bridge, and as you approach the tunnel there are two rieklings on a ledge above you to your right. The tunnel climbs and at the top turns to the north. Ahead you can see light to the left and as you move forward you will see three rieklings standing near a campfire. To the right is a dead end with rocks. Among them is a random mace. To the left, above the campfire is a ledge with riekling fencing around the edge and to the west is a short tunnel leading to a set of unlocked double doors.

Through the doors in a partially flooded tunnel with an ice pillar in front of you, where the tunnel turns to the north. Just after the turn there is a novice-locked door on your right. Inside is a cell with a skeleton, a large coin purse, and a random war axe. The tunnel continues past this and then branches with another unlocked door to the east and around the corner to the left two more, one to the north which is novice-locked and one to the south which is apprentice-locked. Behind the one to the south is a cell with a dead riekling with some riekling spears next to it inside. The one to the north is another cell with a mammoth rib bone and in the corner an empty random soul gem.

Through the door to the east is another tunnel rising and at the top it turns to the south and emerges onto the ledge above the campfire passed previously. There are three barrels on your right with a random sword (see bugs), a random healing potion, and a random potion beside them. To the left the ledge continues into a short tunnel heading east and emerges onto another ledge above the lake. You can see more tents on both sides in front and above you on other ledges. As you emerge a riekling will burst from the snow in front of you and a second one will emerge from the tent. On your right on top of a crate is a random bow, with a quiver of random arrows beside it and a sack behind it. To the left of the tunnel are two barrels and a sack with a tent to the right of them. Behind the tent is a lookout platform, with a riekling standing guard; at the top is a medium coin purse. Straight ahead of the tunnel there is a tent on each side and a bridge across the cavern in front. Instead of crossing stay to the right and follow the ledge around the corner. There are three random soul gems, two of which are filled, on the ground or in an ice bowl leaning forwards, next to a riekling hut (see notes). You are now above the two rieklings on the ledge by the first tunnel.

Bristlebacks

Crossing the bridge leads to three tents, two of which have a riekling inside. On your left as you leave the bridge are two barrels and a sack beside the first tent, ahead against the north wall is the second, this one has a random war axe on the ground and on the right is the third. Behind this one is a riekling hut. There is a tunnel heading up to the east. At the top you enter another cavern with a pile of barrels, crates and sacks by a fence in front of you, more are stacked to the right. Among them is an apothecary's satchel with a random helmet beside it on a crate. Further to the right another tunnel heads back to the west. To the left of the tunnel is a path descending to a lookout platform with a riekling on guard and an open pen with three bristlebacks underneath where the barrels are. There is a waterfall dropping from the ceiling near the southeast wall. Next to the pen are a pile of barrels, crates and sacks, with a random potion on the ground beside them, while to the southwest in the corner is a novice-locked chest with a large coin purse beside it.

Retrace your steps back to the top and take the tunnel to the west. It leads to a door that opens onto another ledge above the original lake below. There is a tent with a riekling inside in front of you, to the right of a bridge across the cavern. On your left is a pile of crates and a riekling barrel, that a riekling will burst from as you move forward. Across the bridge you can see another leading to two tents and a hut, there are also two more rieklings there. To the left of these is a waterfall plunging to the lake below and a second further to the left dropping from a higher ledge. In the tent nearest the hut is a random enchanted weapon and to the left of this tent is an unlocked chest and behind it across a stream is a skeleton that can be looted with a leveled mace and leveled shield beside it. There is a ledge above where three waterfalls are dropping to form the stream before it drops off the edge. You can see another skeleton hanging over the edge up there. This can be dislodged using Unrelenting Forcedragon shout, it can then be looted for a few septims. There is also a chest on the same ledge that can be reached with careful use of the Whirlwind Sprint dragon shout from the far side of the cavern.

Once across the second bridge turn right and jump/climb the ice until you reach another ledge. Proceed forward through a narrow gap to the northeast and there is a set of random armor, a medium coin purse, and an adept-locked chest together on your left. To the west is another pair of doors, open with a tunnel heading up to the west, which then turns to the north, passes a fence with crates, barrels and a sack behind it. Among these things are a leveled potion of resist cold, a leveled healing potion, a pair of leveled gauntlets, and a small coin purse. Past this is an exit to Castle Karstaag Courtyard.